Our 5 yr old Eclectus is biting her leg. Vet can't find a reason.HELP?

For about a year our Eclectus is biting her leg. To a point of bleeding. We have tried just about everything to get her to stop, but once it's almost healed, she starts again. The vet has wrapped the leg, she's been wearing the neck ring of shame. Antibiotics & pain cream.Moved the cage, changed the toys. Blood work and biopsy came back negative for everything the vet was looking for. This is costing a small fortune, but we love Zoe and don't want to get rid of her. Has anyone else had a similar problem? We are desperate!!!